Introduction

Particle induced electron emission from the walls of a plasma device alters the sheath potential adjacent to the walls, this in turn alters plasma transport in the scrape-off layer. Electron emission from the target of a collisionless divertor reduces both the plasma temperature and the sheath potential. The latter effect also lowers the energy of ions incident on the target and hence the sputtering yield and impurity production.
The most important factor in particle induced electron ejection from surfaces is the coefficient for total electron ejection from the surface, defined as the number of electrons ejected per incident particle. This coefficient is needed as a function of particle energy and as a function of incidence angle.

There are three types of processes whereby particles, incident on a surface, give rise to electrons ejected from that surface. The most general is the kinetic process whereby an incident species, electron or heavy particle, transfers part of its kinetic energy to electrons in the target, and some of these electrons have a direction and energy which permit them to escape into free space. The second process is a potential ejection, a process where an electron from the solid transfers its kinetic energy to a lower energy state of the incoming projectile and the excess energy is transferred by an Auger process to a second target electron which escapes. Potential ejection processes are obviously restricted to incident heavy particles which are ionized or excited. The third process is the reflection of incident electrons. Strictly speaking, this is not an ejection process at all, but it is backscattering. Since a backscattered electron cannot be inherently distinguished from a true secondary electron, there is a tradition of including the process together with other aspects of secondary electron emission.

Electron emission by the kinetic process is likely to be the dominant ejection mechanism for most circumstances. At relatively high projectile impact velocities it may be thought of as split into three stages: (1) Projectile interaction with the target excites target electrons to continuum states, creating a flux by primary excitation. Some secondary excitation may also be created by the more energetic excited electrons. (2) A fraction of the excited electrons migrate to the surface. (3) Electrons escape through the potential barrier at the surface and may be detected as ‘secondary’ electrons. This three-step process is independent of whether the incident projectile is an ion or an electron.

The potential ejection process involves electron transfer to vacant states of the incident projectile, followed by an Auger decay process. Being related to vacant states of the projectile, it will occur only for ionized or excited incoming heavy species. The contribution to the coefficient will be independent of energy, and the process will occur down to zero impact velocity. The ejected electrons will have specific energies related to the juxtaposition of energy levels in the projectile and the target. Apart from very low impact energies, the potential ejection process is relatively unimportant and total electron ejection is likely to be dominated by the kinetic ejection process, except for the impact of highly ionized heavy ions.

Incident electrons may be reflected and may contribute to ejection of electrons from a surface. The majority of reflected electrons have energies close to that of the incident projectile electron; by comparison, true secondary electrons have energies that peak in the region of a few electron volts. The distributions do, however, overlap and there is no way of distinguishing reflected electrons from ejected electrons.